The number for lens f/stop in photography (for example, f/8) is the ratio of lens focal length divided by the effective lens aperture. Aperture is not the obvious physical diameter (shown by black vertical lines below), but instead is the apparent "working" diameter as seen through the magnification of the front lens element.
That is math and physics, but still, the very useful purpose of f/stop numbers is the grand concept that f/8 will the same exposure in any lens, regardless of focal length, or physical size of construction. OK, yes, there can be slight minor variations, especially in the old days in lenses without modern coatings (losing a lot of light), and even today in fancy lenses containing many glass elements (losing a slight amount of light). Slight variations, but which are important in professional movie cameras, visible variations when switching lenses on the same scene. So their lenses can use T-stops, which are calibrated to the actual amount of light the lens transmits, instead of the theoretical amount. This is relatively unimportant for still cameras.
And while we are momentarily distracted, the marked Focal Length is when focused at infinity, but it changes as we focus closer (focal length normally becomes longer if front element is extended, but internal focus lenses probably become shorter). The actual focal length is measured to the rear Principle Point, H', as shown above. The Principle Point is the designer's apparent plane where the image appears to be. Design of curved lens elements can move this point, and this H' point is in fact often literally outside the actual lens, either in front or behind. In telephoto lenses, this H' point is always slightly in front of the front lens element, because, the actual optical technical definition of "telephoto" is that the lens is physically shorter than its focal length (which is a practical way to build the long lenses that show distant objects enlarged). Wide angle lenses are often retro-focus, which means the rear node H' is well behind the rear element. This allows the short lens to be mounted well forward, leaving space for the SLR camera mirror to be raised. Otherwise for example, an 18 mm lens would block raising a mirror 24 mm tall. FWIW, regarding this H' distance, the ratio of subject size to image size is called Magnification, and when equal sizes (at 1:1 reproduction ratio), then also equal distances - the distance in front of the lens is necessarily equal to the distance behind the lens, at 1:1 size (similar triangles, etc.) Seems a cute fact, which aids understanding.
Is f/stop written f/stop or f-stop? The lens manufacturers properly write f/8. But the term f-stop has become popular on-line, so we see both. My habit is still to write f/stop, because we write f/8, to be remindful of the division defining it:
f/stop = focal length / aperture diameter.
So, f/8 denotes "focal length / 8", which represents the "effective" aperture diameter of the lens, and this exposure value can be compared with other lenses in this way. Then a series of multiple f/stop steps is designed, called "stops". Each full stop towards larger numbers gives half the light exposure of the previous step (called stopping down, which also increases depth of field).
The tables below are the computed f/stop numbers of the camera aperture. The first table is the fractional steps in tenth stops. These charts show the actual numbers, and the relationships, and one purpose is to aid determining span in stops between two values.
Notes: f/stop = sqrt(2)^(stop number + fraction)
Usage: If you set your light meter to read in tenth stops, the format of the result value shown is (for example):
This is NOT f/8.7. It is 7/10 of the way between f/8 and f/11 - or about f/10. If you set the flash meter to read in third stops, the same light will read f/10.
By definition, both of these equivalent values are simply two third-clicks past f/8, or one third-click below f/11 (easy to set). The camera dial will indicate f/10 there, but we can instead meter and work in tenth-stop differences from full stops.
Fractions: 1/10 is 0.1 stop. The fraction 1/3 stop is 0.33 stops, and 2/3 stop is 0.67 stops, so a reading around 0.3 is one third stop, and one around 0.7 is two third stops. The lens can only be set to third stops, so just pick the nearest third stop: 0, 1/3, 2/3, or 1 stop.
There would seem no point of 1/10 stop meter readings for daylight, since we can only set the camera to third stops. But one good reason to use tenth stops is for greater precision in adjusting the power level of individual flash units - the actual difference between two lights could be controlled more closely. But the overwhelming advantage is when pondering fill level for that lighting ratio - how much is 1.3 stop less than f/10? It is about f/6.3, but who knows that? But if we read this as f/5.6 plus 3/10 stop vs. f/8 plus 6/10 stop, then we easily know 1.3 stops in our heads, immediately. Or, the chart above can determine it too.
Rounding: (values are more often truncated, or even approximated).
We say it as f/11, but f/11.3 is the correct actual calculated value. Most other values are closer, but Guide Number calculations for speedlights ought to use f/11.3 instead of f/11.
To make that fact be obvious, note that progressions arranged into rows of every-other doubled aperture values are also defined as:
|f/||1.41||2.83||5.66||11.31||22.62||45.25||(sqrt(2) is 1.414)|
Marked shutter speed numbers are also approximated. For example, the standard shutter speed chart below shows 1/20 second and 1/10 second (and 10 and 20 seconds) to be both third stop values and half stop values. Same value cannot be both, and the camera does compute the actual value closer (half stop 20 seconds will be 22.6 seconds, and 30 seconds will be 32 seconds), but we humans are frequently shown easier rounded or even "equivalents".
The area of a circle is defined as Pi r² (aperture is circular). Double area is twice the light, or one stop.
For double area: 2 Pi r² = Pi (1.414 x r)² , so 1.414x radius gives one stop. Sqrt(2) is 1.414.
Since f/stop = focal length / aperture diameter, then f/stop numbers increase in 1.414x steps.
Inversely, when the diameter and area are made larger, the f/number from the ratio f/d becomes a smaller number.
Full f/stop numbers advance in 1.414x multiples. From any f/stop number, in all cases, double or half of that number is two stops (for example, f/6.2 is two stops above f/3.1). Every second stop is the doubled f/number. Or one stop is x1.414 or /1.414.
Lens manufacturers seem to truncate instead of round off. For example, f/5.6 is actually 5.66, and f/3.5 is probably 3.56 (if it is a third stop). Less (number) is More (light).
The values of shutter speed and ISO are linear scales, meaning that 2x the number is a 2x difference, and 2x is one stop. But half and third stop numbers are not simply 1.5x or 1.33x, that's wrong. Because - two half stops computed 1.5 x 1.5 comes out 2.25 more, not 2.0. And three third stops computed 1.33 x 1.33 x 1.33 comes out 2.37 more, not 2.0. So instead, square root and cube root steps are the proper values.
The next third-stop increment is cube root of 2 (1.26) greater than the previous value.
Every three third-stop values (from any point) is exactly a 2.0x change of the light.
The next half-stop increment is square root of 2 (1.414) greater than the previous value.
Every two half-stop values (from any point) is exactly a 2.0x change of the light.
The next full-stop value is 2x greater than the previous value. Doubling any numeric value is one stop (speaking of shutter speed or ISO, but 2x number is two stops for f/stops, see above.)
ISO speed was a film sensitivity concept. Digital speed is a gain factor, multiplied after the digital sensor does what it does, but the same ISO numbering scheme is used, still an apparent "sensitivity" indication.
|Shutter Speed||Marked As:|
Shutter speed is of course the time duration when the shutter is open, exposing the sensor or film to the light from the aperture. On many cameras, numerical values for shutter speed are marked on the camera using two methods with different meanings - for example, marked as 4 or 4". Just the number alone, like 4, is an implied fraction (1 over the number), meaning 1/4 second. The same number written 4" means four whole seconds, not a fraction. A slow shutter is a longer duration, and a fast shutter is a shorter duration.
A flash, especially a speedlight flash, is typically a much shorter duration than the shutter. The flash simply must occur while the shutter is open (sync), but the faster flash exposure is not affected by the slower shutter speed. Keeping the shutter open longer does increase the continuous ambient light seen, but shutter speed does not change what the fast flash does.
There are two shutter speed charts below, the camera's normal rounded marked values, and the theoretical computed value goals that are actually used.
Each "Theoretical Actual" shutter speed at right below is just the simple progression (starting at one second), showing third stop times in sequential multiples of cube root of 2 (1.2599), and half stop times as sequential multiples of square root of 2 (1.4142). This insures that every interval of three third-stops, or two half-stops, is exactly 2x or 1/2x value (i.e., exactly one stop). These are the "Actual" shutter speed goals the camera uses. I am not implying practical accuracy is within a microsecond, my goal was merely to show four significant digits for 1/1000 to 1/8000 second values. The goal of precision here is to show that any and every third value of third stops is exactly double or half value (one stop).
The other "Shutter Speed" chart is the camera's normal marked numbers for same shutter speeds. These marked values take liberties to show even or round values for convenient human use. This is just a marking, which does not affect what the shutter does. It is difficult to verify the fast numbers, but at the 30 second end, we can easily measure and confirm the camera shutter in fact does use the computed theoretical numbers (32 seconds actual instead of the marked 30 seconds). It must do that, because the basis of the system is that one stop is 2x the light. The markings only have about 2% or 6% numeric discrepancy (the 10 and 20 half-stop values are a little worse), which is insignificant, and really, only in our own minds, not in what the shutter actually does. Cameras could mark the actual values, but we are used to this system (and it is convenient). The precise marking is not very important, the important need is for each stop to be 2x the light from previous stop - easy work for today's electronically timed shutter. So the point is NOT that there is a marking discrepancy, but instead is that you need not be concerned about it. The shutter does the right thing, and it really is a pretty neat system, better than our learning a sequence of numbers like 1/813 second shutter speed. All we need to know is that double or half is one stop, and these marked rounded numbers make it easy to know the next one.
F/stop, ISO, Shutter Speed Values - in Full, Third and Half stops
EV is an "exposure", but EV is simply about combinations of numerical camera settings, regardless if it is an accurate exposure or not. EV is not a measurement of the light. It may represent a capability, but it is only about your numerical camera settings. EV basically gives a name to the group of several "equivalent exposure" choices in any one row below. Each row is a one stop step from its adjacent rows. A 1 EV step is one stop. A "stop" is a 2x or 1/2x exposure change, regardless if due to aperture, shutter speed, or ISO.
So EV is independent of ISO, it is merely about the numerical settings on the camera. Light meters often have a mode to meter an EV value (a row in the chart), for the ISO value you select. Different ISO values will meter different EV values - BECAUSE, while the light may be the same, the results have different apertures or shutter speeds. EV is basically the combinations of the numerical camera settings, and represents a capability of the settings, and not necessarily an accurate exposure. It is NOT about the light seen.
However, that said, some uses do publish EV as if it refers to an illumination level at ISO 100. For example, the Nikon DSLR cameras specifies their metering exposure range as 0-20EV, qualifying that these numbers are true if with a f/1.4 lens at ISO 100. Or for example, Sunny 16 in bright sun is near EV 15 if ISO 100 is assumed, but it will have other EV values at other ISO. If it is said that the statement applies only to ISO 100, then it does.
Mathematically, EV = Log2 (f²/T). It is much easier to create this table however. By definition, EV 0 is f/1 at 1 second, and all values are double the adjacent value. The chart shows camera shutter speeds, in seconds and minutes.
|-5||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m||512 m||1024 m||2048 m|
|-4||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m||512 m||1024 m|
|-3||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m||512 m|
|-2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m||256 m|
|-1||2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m||128 m|
|0||1||2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m||64 m|
|1||1/2||1||2||4||8||15||30||60||2 m||4 m||8 m||16 m||32 m|
|2||1/4||1/2||1||2||4||8||15||30||60||2 m||4 m||8 m||16 m|
|3||1/8||1/4||1/2||1||2||4||8||15||30||60||2 m||4 m||8 m|
|4||1/15||1/8||1/4||1/2||1||2||4||8||15||30||60||2 m||4 m|